A heat exchanger mounted in an apparatus such as a water heater, a heat source device for a room heater and the like has a case body in which combustion exhaust gas flows, a plurality of heat-transfer pipes arranged in a vertical direction and a horizontal direction inside the case body, and a header provided in a side wall outer surface of the case body. Open ends of the plurality of heat-transfer pipes communicate each other in the header. In this type of heat exchanger, water needs to be drained from insides of the heat-transfer tubes in order to prevent freezing of the water inside the heat-transfer tubes in winter. There has been proposed a technique of disposing a drainage plate made of punching metal facing an open end of each of the heat-transfer tubes inside the header so that the water is prevented from remaining in a vicinity of the open end of the heat-transfer tube and the water is smoothly drained from an inside of the heat-transfer tube. (For example, Japanese Unexamined Patent Publication No. 2015-114003 A)
However, the drainage plate made of the punching metal used in the above-described conventional heat exchanger has punched holes of a smaller diameter than that of the heat-transfer tube at the open end. This poses a problem that the open end of the heat-transfer tube is partially closed by the drainage plate, so that a pressure loss in a water passage of the heat-transfer tube becomes large during normal operation. Particularly, when the diameter of the heat-transfer tube is reduced, the pressure loss due to the drainage plate becomes larger. Moreover, in the case where the above-described drainage plate is used in a sensible heat exchanger having a relatively high temperature in the heat-transfer tube, as compared with a latent heat exchanger, there is a possibility that the punched holes are clogged with deposit such as calcium and the like deposited inside the heat-transfer tube.